(color online) Thermal management of NWs. (a) Schematic drawing of the graded Si NWs. (b) Rectification ratio (left vertical axis) and normalized S + / S − of phonon density of states overlap ratio (right vertical axis) as function of angle. (c) SEM images of short-CuNW–polyacrylate composites. (d) Experimental thermal conductivity of metal NW–polyacrylate composites with different volume fractions at 298 K. Panels (a) and (b) adapted with permission from Ref. [128], ©2016 Elsevier; (c) and (d) from Ref. [137], ©2014 ACS.